Photovoltaic cell based on zinc oxide nanorods and method for making the same

ABSTRACT

A new photovoltaic (PV) cell structure, prepared on transparent substrate with transparent conductive oxide (TCO) layer and having nanorod zinc oxide layer. The cell has a thin conductive layer of doped zinc oxide deposited on the nanorod zinc oxide layer, an extremely thin blocking layer of titanium oxide or indium sulfide on the thin conductive layer, a buffer layer of indium sulfide on the extremely thin blocking layer, an absorber layer, comprising copper indium disulfide on said buffer layer and one electrode attached to the transparent conductive oxide layer and a second electrode attached to the absorber layer. Also, a method of preparing a zinc oxide nanorod PV cell entirely by chemical spray pyrolysis is disclosed. Efficiency up to 3.9% is achieved by simple continuous non-vacuum process.

BACKGROUND OF THE INVENTION

1. Technical Field

The invention relates to photovoltaic cells and methods of makingphotovoltaic cells, particularly to methods of manufacturingphotovoltaic cells on ZnO nanorod structures, whereas all layers of thenanorod structure are preferably prepared by chemical spray pyrolysis.

2. Background Art

Photovoltaic (PV) cell is a device that converts light energy intoelectrical energy. Harnessing solar energy with inexpensive materialsand manufacturing methods is an important challenge. Low cost depositiontechniques and new designs of PV devices are needed to reduce theproduction costs. There has been much interest of using nanostructuresin PV devices. Dye sensitized photoelectrochemical solar cell (DSSC)based on nanoporous titanium dioxide is the most known nanostructured PVdevice (B. O'Regan and M. Grätzel, Nature 353, 737 (1991)). Unsolvedproblem with DSSC is its instability, also of its solid-statemodifications. Another approach is an extremely thin absorber (eta) cellwhich has an extra thin absorber sandwiched between two stronglyinterpenetrating transparent wide band gap semiconductors (K. Ernst, etal, Semicond. Sci. Technol. 18, 475 (2003)). Most frequently used n-typenanostructured window material for the eta-solar cell is porous TiO₂.Alternatively, ZnO nanowires or columnar ZnO structures have been usedto prepare ZnO eta-cells (C. Lévy-Clément, et al, Physica E 14, 229(2002)). Inorganic absorber materials like CdTe (C. Lévy-Clément, et alabove; R. Tena-Zaera, et al, Thin Solid Films 483, 372 (2005)), CdSe(Lévy-Clément, et al, Advanced Materials 17, 1512 (2005); R. Tena-Zaera,et al, C. R. Chimie 9, 717 (2006); R. Tena-Zaera, et al Proceedings 21stEuropean PV Solar Energy Conf., 4-8 Sep. 2006, Dresden, Germany (2006),p.238) or In₂S₃ (D. Kieven et al, Applied Physics Letters 92, 153107(2008) have been used in ZnO based cells. The conversion efficiencies of2.3-2.5% are reached in ZnO nanowire based eta-cells (see C. R. Chimie,above; D. Kieven et al, above).

ZnO nanowire layers for photovoltaic applications have been fabricatedby electrodeposition (see C. Lévy-Clément, R. Tena-Zaera above),metalorganic vapour deposition (J. B. Baxter and E. S. Aydil, Sol.Energ. Mater. Solar Cells 90, 607 (2006)), hydrothermal growth (M. Guo,P. Diao, X. Wang and S. Cai, J. Solid State Chem. 178, 3210 (2005) andsolution deposition (D. Kieven et al, above).

In US patent application to Yang et al (Publication No. US2005/0009224A1) is described a method of growing zinc oxide nanowires(aspect ratios between about 10 to about 500) on transparent conductiveoxide (TCO) covered substrate, such as glass, and dye sensitized solarcells, organic-inorganic solar cells and solid state sensitized solarcells built on such nanowires. The nanowires in Yang are deposited bysolution based processes, e.g., by dip coating process.

Recently we have developed a low-cost deposition method of growing zincoxide nanorod arrays on conductive transparent electrodes by chemicalspray (M. Krunks, et al, U.S. provisional application 60/671232;international patent application PCT/EE2006/000002, published asWO2006108425).

DISCLOSURE OF THE INVENTION

Embodiments of the invention are directed to novel structures of aphotovoltaic (PV) cell, based on nanorod layer, and methods for makingthe same.

One aspect of the invention is a new PV cell, comprising a transparentsubstrate covered with transparent conductive oxide (TCO) layer, ananorod metal oxide layer on said TCO layer, a (chemically) blockinglayer on said nanorod metal oxide layer, a buffer layer on said blockinglayer, an absorber layer on said buffer layer, and electrical contactsattached to said absorber layer and to said TCO layer.

According to one embodiment, the nanorod metal oxide layer is a ZnOnanorod layer. According to one embodiment, the ZnO nanorod layer isdeposited by spray from solution containing ZnCl₂.

According to one embodiment, the transparent substrate is glass, and theTCO layer is an indium tin oxide (ITO), doped SnO₂, or doped ZnO layer.

According to one embodiment, the extremely thin blocking layer comprisesTiO₂ and has thickness less than 10 nm, preferably less than 5 nm.According to one embodiment, the extremely thin blocking layer comprisesIn_(x)S_(y). According to one embodiment, the buffer layer comprisesIn₂S₃, CdS or ZnS. According to one embodiment, the absorber layercomprises CuInS₂, or other Cu-based chalcopyrites such as CuInS₂,CuInSe₂, CuInGaS₂, CuInGaSe₂ and their solid solutions, or analogousAg-based compounds and their solid solutions; or In-free CZTS-typecompounds, such as Cu₂ZnSnS₄, Cu₂ZnSnSe₄ and/or their solid solutions.

According to one embodiment, the PV cell further comprises a thinconductive layer between said nanorod metal oxide layer and saidblocking layer. According to one embodiment, said conductive layer is adoped metal oxide layer, such as indium or aluminium ZnO layer.

One embodiment of the invention is a PV cell, comprising a glasssubstrate covered with an ITO layer, a nanorod zinc oxide layer,deposited by spray from solution containing ZnCl₂; an indium-doped zincoxide layer, deposited by spray from a solution comprising zinc acetateand indium ions (In³⁺ ions); a blocking layer, prepared by dip coatingor by spray from titanium alkoxide sol; a buffer layer comprising In₂S₃and prepared by spray; and absorber layer, comprising CuInS₂, preparedby spray.

Another aspect of the invention is a method for manufacturing PV cellswith structures as described above. Such structures are prepared solelyby or mostly by chemical spray pyrolysis deposition.

According to one embodiment, the method comprises depositing a metaloxide, such as ZnO nanorod layer by chemical spray deposition on atransparent conductive oxide layer on a transparent substrate;depositing an extra thin blocking layer on said nanorod layer, saidextra thin blocking layer comprising TiO₂ or In_(x)S_(y) (where x and yare integer numbers); depositing a buffer layer on said thin blockinglayer, said buffer layer comprising In₂S₃; and depositing an absorberlayer on said buffer layer, said absorber layer comprising CuInS₂; andattaching electrical contacts to said transparent oxide layer and tosaid absorber layer.

According to one embodiment, the metal oxide nanorod layer is depositedby spray from solution containing ZnCl₂.

According to one embodiment, the method additionally comprises a step ofdepositing a conductive doped metal oxide layer on said metal oxidenanorod layer.

BRIEF DESCRIPTION OF THE DRAWINGS

The technical essence of the invention is described in details byfollowing figures.

FIG. 1 shows a simplified schematic view of a photovoltaic cellaccording to the invention;

FIG. 2 shows SEM images of a ZnO nanocolumnar layer before and after(inset) spraying on that acidic (pH ˜2.8) solution (FIG. 2A) and a solarcell structure ZnO_(R)/TiO₂/In₂S₃/CIS (FIG. 2B).

FIG. 3A shows a flowchart illustrating the method of manufacturing thePV cell according to one embodiment of the invention.

FIG. 3B shows a flowchart illustrating the method of manufacturing thePV cell according to another embodiment of the invention.

FIG. 4A is a current-voltage characteristics of a PV cell according tothe invention (structured cell) compared to a flat PV cell(respectively, cell 1 and cell 2 shown in Table 1) under halogen lampillumination of 100 mW/cm2.

FIG. 4B is a current-voltage characteristics (in dark and under theillumination) of all-layers-sprayed PV cellITO/ZnO_(R)/TiO2/In2S3/CuInS2 with a cell conversion efficiency of thecell 2.6% (cell 4 shown in Table 1).

FIG. 4C is a current-voltage characteristics of structured PV cells withdifferent thickness of spray-deposited TiO2 layer. Thickness iscontrolled by number of the spray pulses shown by Arabic numerals on thegraph.

FIG. 4D is a current-voltage characteristics (in dark and under theillumination) of all layers sprayed structured PV cellITO/ZnO_(R)/In_(x)S_(y)/In₂S₃/CuInS₂ with a cell conversion efficiency3.9% (cell 6 shown in Table 1).

FIG. 5 shows electron beam induced current (EBIC) (FIG. 5A) and SEM(FIG. 5B) images of the cross section of a structured solar cellTCO/ZnO_(R)/ZnO:In/TiO₂/In₂S₃/CIS.

FIG. 6 is a spectral response of a PV cell according to the invention(structured cell) compared to a flat PV cell (respectively, cell 1 andcell 2 shown in Table 1).

MODES FOR CARRYING OUT THE INVENTION

The photovoltaic cell (PV cell, or solar cell) according to oneembodiment of the present invention is schematically depicted in FIG. 1.The PV cell is built on suitable transparent substrate 1 that is coveredwith transparent conductive oxide (TCO) layer 2. Glass is one of themost suitable materials for the substrate. TCO layer is typically tinoxide, e.g., fluorine-doped tin oxide (SnO₂:F, or FTO), or indium tinoxide (ITO) or indium, fluorine or aluminum-doped zinc oxide (ZnO:In,ZnO:F, or ZnO:Al). TCO-covered glass is commercially available from manymanufacturers.

A nanorod layer 3, such as zinc oxide nanorod (ZnO_(R)) layer isdeposited on said TCO substrate. The nanorod layer is preferablyprepared by chemical spray pyrolysis deposition (hereinafter, spray).ZnO layer comprises elongated crystals. According to selective areaelectron diffraction (SAED) study the sprayed ZnO nanorods grown attemperatures above 500° C. are single crystals. According to thephotoluminescence studies which shows strong near-band-edge (NBE)emission in UV region and very weak green emission, the sprayed ZnOnanorods are of high crystal quality and chemical purity.

Electrical characterization, incl Kelvin probe measurements reveal thatZnO nanorods by spray may be single crystals with low concentration offree carriers. For better carrier collection, a thin conductive metaloxide layer 4, such as indium-doped zinc oxide (ZnO:In) layer isdeposited on the nanorod layer 3 and generally follows the shape of therods. The conductive layer 4 is also preferably prepared by spray. Inbackground art, the ZnO rods, typically prepared by electrodeposition,are heavily doped.

An extra thin blocking layer 5 (with thickness less than 50 nm,preferably less than 10 nm, most preferably less than 5 nm) is depositedon said conductive layer. Blocking layer 5 can comprise TiO₂ and can bemade by sol-gel spray or spin coating or dip coating using a titaniumalkoxide based sol. Other chemically inert oxides such as Al₂O₃, ZrO₂and Nb₂O₅ can be also used. The blocking layer protects the ZnO fromchemical dissolution in acidic medium during next deposition steps ofsolar cell fabrication, namely during the spray deposition of acidic(pH˜3) solution to make In₂S₃ buffer and copper indium disulfide (CIS)absorber layers. Blocking layer also avoids the electrical shortcircuiting of the solar cell structure. The thickness of the blockinglayer should be optimized to provide tunneling of the carriers.

The blocking layer 5 may comprise indium sulfide or titanium dioxide andmay be deposited by spray. This reduces the preparation time andpreserves continual spray process and thus, provides simple andstraightforward solar cell fabrication process. According to the SEMstudy, thin, dense and pinhole-free films of InS or TiO₂ can be formedby spray.

The buffer layer 6 is deposited on the blocking layer 5. Buffer layermay comprise In₂S₃ and is preferably deposited by spray. Absorber layer7 is deposited on buffer layer. Absorber layer is preferably CIS(CuInS₂) layer, preferably deposited by spray. However, other chemicalvapor deposition and solution based techniques may be also used. Alsoother absorber materials may be used, such as other Cu-basedchalcopyrites such as CuInSe₂, CuInGaS₂, CuInGaSe₂ and their solidsolutions, or suitable Ag-based materials and their solid solutions; orIn-free multinary compounds, CZTS, such as Cu₂ZnSnS₄, Cu₂ZnSnSe₄.

The solar cell has electrodes attached to the p-type absorber layer as aback contact 8 and to transparent conductive oxide layer as a frontcontact 9. For the back contact any suitable method and materialcommonly used for electrodes can be used, e.g., metals with high workfunction Co, Au, Ni, Pd, Pt or graphite or hole conductor layerPEDOT:PSS, CuSCN, CuI, CuAlO₂, NiO with a contact formed of suitablemetal such as Co, Au, Ni, Pd, Pt.

FIGS. 3A and 3B show flowchart, illustrating the methods ofmanufacturing PV cells according to the embodiments of the invention.

The method shown in FIG. 3A comprises step 100 of depositing a nanorodlayer 3 of metal oxide by spray on suitable TCO substrate, such as ITOcovered glass. Metal oxide is preferably zinc oxide. Then follows step300 of depositing an extra thin blocking layer 5 on said nanorod layer,covering the tops and sides of the rods. The blocking layer ispreferably also deposited by spray and comprises In_(x)S_(y) (where xand y are integer numbers), or TiO₂. Follows step 400 of depositing abuffer layer 6 on the blocking layer 5. The buffer layer may compriseIn₂S₃ and is also preferably deposited by spray. Then follows step 500of depositing absorber layer 7 on the buffer layer. Absorber layer ispreferably CIS (CuInS₂) layer, and is preferably deposited by spray. Thefinal step 600 is attaching suitable electrodes to the transparentconductive oxide layer 2 and to the absorber layer 7.

The nanorod layer prepared by spray consists of single crystals whilesuch crystals may be, depending on the deposition parameters, of veryhigh purity. To improve the carrier collection in such PV cell,additional conductive layer may be needed between the nanorod layer andthe blocking layer. To manufacture such PV cells, the method is modifiedas shown in FIG. 3B. The method additionally comprises step 200 afterstep 100. Step 200 is depositing a thin conductive layer 4 of dopedmetal oxide, such as indium or aluminium doped zinc oxide onto thenanorod layer 3 by spray, covering the tops and the sides of the rods.Then follows step 300 of depositing an extra thin blocking layer 5 onsaid thin conductive layer. Other steps 400, 500 and 600 are asdescribed above.

Example 1

Zinc oxide (ZnO) nanorods were deposited by spray of zinc chloride(ZnCl₂) aqueous solution onto indium tin oxide (ITO) covered glasssubstrates placed on the hot plate (laboratory device developed byTallinn University of Technology) heated up to about 600° C. Theconcentration of ZnCl₂ in spray solution was about 0.1 mol/l. ZnOnanorod (ZnO_(R)) layers deposition by spray technique are described inmore details in our PCT application PCT/EE2006/000002, published asWO2006108425.

The next layers of the solar cell were deposited in the following order:a thin conductive layer of indium doped zinc oxide (ZnO:In), an extrathin blocking layer TiO₂, a buffer layer In₂S₃ and finally, CuInS₂ (CIS)absorber layer.

The conductive layer of indium-doped zinc oxide (ZnO:In) was depositedonto the ZnO nanorods at hot plate temperature of about 500° C. fromabout 20 ml of about 0.2 mol/l Zn(CH₃COO)₂ solution containing InCl₃([In]/(Zn]=3 at %).

The extra thin blocking layer of TiO₂ with thickness less than or about10 nm was prepared by sol-gel dip coating method by immersing thesubstrate in the titania sol (acetylacetone stabilisedtitaniumtetraisopropoxide, prepared at TTIP:acacH=1:1 in ethanol (otheralcohols, such as isopropanol, 2-metoxyethanol may be used) whereTTIP=titaniumtetraisopropoxide, C₁₂H₂₈O₄Ti and acacH=acetylacetone,C₅H₈O₂). Dip coating was made at room temperature followed by drying atabout 80° C., and then heated for about 30 minutes at about 450° C. in alaboratory oven.

Indium sulfide (In₂S₃) buffer layer was deposited by spray using anaqueous spray solution of InCl₃ and SC(NH₂)₂ with molar ratio ofIn:S=1:3 at concentration of InCl₃ of 2×10⁻³ mol/l and pH ˜3.

CuInS₂ (CIS) absorber layer was deposited by spray using a solutioncontaining InCl₃, CuCl₂ and SC(NH₂)₂ at molar ratios of Cu:In:S=1:1:3and following the deposition route described in details in our US patentapplication published as US20050271827. Indium sulfide layers and CISabsorber layer were deposited at similar temperature of 300° C.

For comparison, flat PV cells (i.e., with flat ZnO layer instead ofZnO_(R) layer) were prepared simultaneously with the structured samples.As can be seen from FIG. 4 and from Table 1, examples 1 and 2, thestructured PV cell has substantially higher current j and efficiency.

Example 2

Zinc oxide (ZnO) nanorods with length of about 1 micron were depositedas in Example 1.

TiO₂ films were deposited by sol-gel spray pyrolysis method onto thesubstrate with ZnO rods using a sol composed of a titanium alkoxide(titanium (IV)isopropoxide) with concentration 0.1 mol/l and astabilizer (acetylacetone) at molar ratio of 1:2 to 1:4 in ethanol(other alcohols may be used). The sol was pulverized onto the substrateheated up to 450° C. employing 2 to 20 spray pulses (1 second spray+30second pause). Sprayed TiO₂ films were amorphous according to Ramanspectra. X-ray photoelectron spectroscopic study revealed that fourspray pulses had produced a continuous and pinhole free TiO₂ film withthe thickness of less than 5 nm on planar surfaces.

Indium sulfide (In₂S₃) buffer layer and CuInS₂ (CIS) absorber layer weredeposited as in Example 1.

TiO₂ film from 2-4 spray pulses forms a chemical blocking layer on ZnOrods resulting simultaneously in reduction of the electrical shortcircuits between front and back contacts and sufficient tunneling of thecharge carriers through the interface barrier. Applying thicker TiO₂films led to S-shaped I-V curves of the solar cells (see FIG. 4C),accompanied by a drastic decrease in the cell fill factor and efficiencydue to an additional rectifying interface in the circuit. The bestall-layers-sprayed ZnO_(R)/TiO₂/In₂S₃/CIS cell shows conversionefficiency of 2.6% (Voc=450 mV, j=11 mA/cm2, FF=54%) versus 1.6% of theflat PV cell under white light illumination of 100 mW/cm2. I-V curves ofthe structured solar cell with sprayed TiO₂ layer from 4 spray pulses ispresented in FIG. 4B.

Example 3

Zinc oxide (ZnO) nanorod layer was deposited by spray. 50 ml of ZnCl₂aqueous solution with concentration of 0.07 mol/l with pH of 2.0-2.2 wassprayed at the rate of 2.5 ml/min onto pre-heated ITO electrode coatedglass substrates kept at constant temperature of about 600-620° C.Acidity of the solution was adjusted via addition of HCl into theaqueous solution of ZnCl₂. The substrates were continuously rotated toobtain uniform layers. The air was used as carrier gas with air flowrate 8 l/min.

Using acidic spray solution with pH of around 2 instead of 5 supportsthe formation of a layer composed of ZnO nanorods, i.e., elongatedcrystals instead of a compact layer of ZnO. The use of acidic solutionreduces the number of ZnO nucleation centers by dissolving the smallernucleation centers and allowing rods to grow on bigger centers withoutgrowing together. Using acidic spray solution makes the process muchless dependant on the surface properties of the TCO layer and thus makeseasier to find suitable TCO substrates for manufacturing PV cells. Also,ZnO nanorods grown from acidic solution are more conductive thannanorods from non-acidic solution.

Thin, compact and dense layer of In_(x)S_(y) was deposited on the ZnOnanorod layer by spray using 25 ml of the spray solution containingInCl₃ and thiocarbamide SC(NH₂)₂ at molar ratio of In:S=1:3 with InCl₃concentration of 4×10⁻⁴ mol/1 and solution pH ˜5, solution spray rate ofabout 1 ml/min, the substrate temperature was kept constant at about300° C. In_(x)S_(y) is composed of In and S atoms and there is no oxygenin the layer according to the X-ray photoelectron spectroscopy. Band gapof In_(x)S_(y) is 2.0 eV, assuming indirect transitions, and thus,similar to that of In₂S₃. Layer of In_(x)S_(y) on ZnO rods is amorphousaccording to Raman spectroscopy; extremely thin layer of In_(x)S_(y) iscompact and without pinholes and covers uniformly ZnO rods according toSEM study.

In₂S₃ buffer layer was deposited as in Examples 1 and 2.

CuInS₂ absorber layer was deposited as in Examples 1 and 2.

Conductive carbon paste was used to make a back contact to CuInS2absorber. Carbon paste contacts with determined area were prepared,solvent was removed by heating the contacts for 60 minutes at 200° C. inair. Our best cell showed the conversion efficiency of 3.9% (Voc=457 mV,j=14.1 mA/cm2, FF=60.3%) under the white light illumination 100 mW/cm2.I-V curves of the solar cell in dark and under the illumination arepresented in FIG. 4D.

Table 1 shows output characteristics of flat and structured solar cellsunder the halogen lamp illumination with intensity of 100 mW/cm2, wherecell No denotes:

1—TCO/ZnO:In/TiO₂(by dip)/In₂S₃/CIS (flat);

2—TCO/ZnO_(R)/ZnO:In/TiO₂ (by dip)/In₂S₃/CIS (structured);

3—TCO/ZnO:In/TiO₂ (by spray)/In₂S₃/CIS (flat);

4—TCO/ZnOR/ZnO:In/TiO₂ (by spray)/In₂S₃/CIS (flat

5—TCO/ZnO/In_(x)S_(y)/In₂S₃/CIS (flat);

6—TCO/ZnO_(R)/ZnO:In/In_(x)S_(y)/In₂S₃/CIS (structured).

TABLE 1 Chemically Cell Cell Blocking Voc, j, No. structure layer mVmA/cm² FF, % Eff., % 1 flat TiO₂ (dipping) 445 5.5 41 1.0 2 Structured ″425 12.0 43 2.2 3 Flat TiO₂ (spray) 440 6.2 58 1.6 4 Structured ″ 45011.0 54 2.6 5 Flat In_(x)S_(y) 485 4.3 63 1.3 6 Structured In_(x)S_(y),no ann. 420 12.8 53 2.9 In_(x)Sy, annealed 457 14.1 60 3.9

Although this invention is described with respect to a set of aspectsand embodiments, modifications thereto will be apparent to those skilledin the art. The foregoing description of the embodiments of theinvention has been presented for the purposes of illustration anddescription. It is not intended to be exhaustive or to limit theinvention to the precise form disclosed. Many modifications andvariations are possible in light of this disclosure. It is intended thatthe scope of the invention be limited not by this detailed description,but rather by the claims appended hereto.

1. A photovoltaic cell structure, comprising: a transparent substratecovered with a transparent conductive oxide layer; a nanorod zinc oxidelayer, deposited to said transparent conductive oxide layer; anextremely thin blocking layer shelling said nanorods of said zinc oxidelayer, said blocking layer comprising TiO₂ or In_(x)S_(y), wherein x, yare integer numbers; a buffer layer on said extremely thin blockinglayer, said buffer layer comprising a material selected from a group ofIn₂S₃, CdS and ZnS; an absorber layer on said buffer layer, saidabsorber layer comprising a material selected from the group of CuInS₂,CuInSe₂, CuInGaS₂, CuInGaSe₂, Cu₂ZnSnS₄ and Cu₂ZnSnSe₄; and a pair ofelectrodes, the first electrode attached to said transparent conductiveoxide layer and the second electrode attached to said absorber layer. 2.A photovoltaic cell structure as in claim 1, comprising a thinconductive layer of doped zinc oxide deposited on said nanorod zincoxide layer.
 3. A photovoltaic cell structure as in claim 2, whereinsaid thin conductive layer is indium doped zinc oxide layer.
 4. Aphotovoltaic cell structure as in claim 1, wherein said transparentsubstrate is glass, transparent conductive oxide is selected from thegroup of indium tin oxide, fluorine-doped tin oxide, and indium-,fluorine- or aluminum-doped zinc oxide.
 5. A photovoltaic cell structureas in claim 4, wherein all layers of the photovoltaic cell are preparedby chemical spray deposition.
 6. A photovoltaic cell structure as inclaim 1, wherein said extremely thin blocking layer comprising TiO₂ hasthickness less than 10 nm.
 7. A photovoltaic cell structure as in claim1, wherein said extremely thin blocking layer comprising TiO₂ hasthickness less than 5 nm.
 8. A method of manufacturing a photovoltaiccell, comprising: depositing a zinc oxide nanorod layer by chemicalspray deposition onto a transparent substrate covered with transparentconductive oxide layer; depositing a conductive layer of doped zincoxide by chemical spray deposition onto said zinc oxide nanorod layer;depositing an extremely thin blocking layer by chemical spray depositiononto said doped conductive zinc oxide layer for chemically protectingsaid zinc oxide nanorod layer during the next steps of manufacturing,said extremely thin blocking layer comprising TiO₂ or In_(x)S_(y),wherein x, y are integer numbers; depositing a buffer layer by chemicalspray deposition onto said extremely thin blocking layer, said bufferlayer comprising In₂S₃; depositing an absorber layer by chemical spraydeposition onto said buffer layer, said absorber layer comprisingCuInS₂; and attaching electrical contacts to said transparent oxidelayer and to said absorber layer, wherein all layers are deposited bychemical spray deposition.
 9. A method of manufacturing a photovoltaiccell, comprising: depositing a zinc oxide nanorod layer by chemicalspray deposition onto a transparent substrate covered with transparentconductive oxide layer; depositing an extremely thin blocking layer onsaid zinc oxide nanorod layer for chemically protecting said zinc oxidenanorod layer during the next steps of manufacturing, said extremelythin blocking layer comprising TiO₂ or In_(x)S_(y), wherein x, y areinteger numbers; depositing a buffer layer on said thin blocking layer,said buffer layer comprising a material selected from a group of CdS,ZnS and In₂S₃. depositing an absorber layer on said buffer layer, saidabsorber layer comprising a material selected from the group of CuInS₂,CuInSe₂, CuInGaS₂, CuInGaSe₂, Cu₂ZnSnS₄ and Cu₇ZnSnSe₄; and attachingelectrical contacts to said transparent oxide layer and to said absorberlayer.
 10. A method as in claim 9, wherein said zinc oxide nanorod layeris deposited onto said transparent conductive oxide layer by chemicalspray deposition from a solution comprising a Zn precursor and a solventwherein said precursor is selected from a group of ZnCl₂ and Zn(CH₃COO)₂and said solvent comprises H₂O.
 11. A method as in claim 10, wherein anacid is added into said solution to adjust a pH level of said solutionbetween 1.5 to 2.5.